This undated file picture shows Osama bin Ladin speaking at an undisclosed place inside Afghanistan. The Al-Qaeda mastermind was killed late on May 1, 2011, by U.S. forces deep inside Pakistan. (AFP/Getty)

Hamza bin Laden has been on the radar of U.S. counterterrorism officials since they found letters in Osama's Abbottabad, Pakistan, compound indicating that he was being groomed for a bigger role in al-Qaeda.

Following the 9/11 attacks, he and his siblings — Osama bin Laden is believed to have fathered between 20 and 26 children — were sent to Iran along with their mothers to avoid the inevitable Western reprisals. Hamza lived there in a sort of protective custody until 2009, when he and two of his brothers left to try and join al-Qaeda's jihad.

Authorities know that Hamza and his father exchanged letters, but don't believe that they were ever reunited after 2001. And no one is quite sure of the bin Laden heir's whereabouts since he left Iran, with various reports putting him in Syria, Pakistan, or the tribal lands along the Afghanistan frontier.

In the years since, Hamza had regularly posted speeches and videos, calling for attacks against Americans, Jews and Western targets, and making frequent appeals for Muslim youth to join the jihad. Al Qaeda appears to be trying to make him the fresh face of a rapidly aging terror movement that has been largely eclipsed by the rival Islamic State.

A screen grab from an undated handout video made available in 2018 by the Central Intelligence Agency (CIA) shows Hamza bin Laden, the son of late al-Qaeda leader Osama bin Laden. (EPA-EFE)

But Hamza doesn't seem to have much street cred beyond his last name, and has yet to be linked to any specific attack.